To define the functions of genes and cell types in vivo, there is no substitute for the power of modifying the mouse germline to generate gene knockouts (KO) and mutants or to express new genes transgenically. For optimal interpretation, genes need to inactivated conditionally or only in certain tissues, or expressed as transgenes in a tissue specific manner in the native genomic context. Using these techniques, investigators worldwide and particularly at Yale, have already been making numerous KO, knockin, transgenic, etc. mouse strains and breeding the modified alleles onto useful genetic backgrounds. These strains, many of which are published, are invaluable resources for research in Rheumatologic diseases, but the maintenance of them is time consuming and expensive, and one cannot envision the endless collection of more and more strains of actively breeding mice. Moreover, infection and breeding problems threaten the existence of many strains, and make the transfer among investigators cumbersome an costly. At the same time, to date the vast majority of engineered mutations have been made by a relatively small number of investigators; these technologies need to be more widely available so that labs that focus on particular genes or processes can have direct access to modifying them, instead of relying on labs that specialize in making KO mice. An important barrier to the technology is in the actual design of KOs and making of constructs as well as construction of BAC constructs via homologous recombination. To address these issues, Core B will take two approaches. First, the Core will provide cryopreservation (and reconstitution of mice from frozen embryos) capabilities for the cost-effective storage of genetically modified mice. Additionally, the core will initiate cryopreservation of sperm and the techniques of IVF for inbred mouse lines. This will produce substantial decreases in costs associated with preservation of mouse strains and will enable distribution both among YRDRCC investigators and outside of Yale. Second, to make modern engineering accessible to the majority of YRDRCC member, the core will provide technological expertise in the generation of KO, conditional KO, knock in, BAC and regulated transgenic constructs. The core will also provide strategies for breeding, genotyping, and initial screening of the resultant mice.